Now RoundCube is Beta Version: 0.1 I tried with ISPConfig and it work fine. I only could validate username, not username@domain. But I think that is possible, RoundCube can read a virtusertable. I am going to try to find a solution!

// automatically create a new RoundCube user when log-in the first time.
// a new user will be created once the IMAP login succeeds.
// set to false if only registered users can use this service
$rcmail_config['auto_create_user'] = TRUE;

// the mail host chosen to perform the log-in
// leave blank to show a textbox at login, give a list of hosts
// to display a pulldown menu or set one host as string.
// To use SSL connection, enter ssl://hostname:993
$rcmail_config['default_host'] = 'localhost';

// Query to resolve user names and e-mail addresses from the database
// %u will be replaced with the current username for login.
// The query should select the user's e-mail address as first col
$rcmail_config['virtuser_query'] = '';

// use this host for sending mails.
// to use SSL connection, set ssl://smtp.host.com
// if left blank, the PHP mail() function is used
$rcmail_config['smtp_server'] = 'localhost';

I don't know the RoundCube package, but I guess there must be a configuration file somewhere (maybe config.php, config.inc.php or something like that). Maybe you can also configure this through the RoundCube web interface.